This invention relates to a method of manufacture for a radiation resistant power MOSFET and the resulting power MOSFET device.
Metal oxide semiconductor field effect transistors ("MOSFETs") are well known. One well known MOSFET is manufactured and sold by the International Rectifier Corporation of El Segundo, Calif., (the Assignee of the present application), under its Registered Trademark HEXFET.RTM.. The structure of such power MOSFETs and a method of manufacture therefor is shown in U.S. Pat. No. 4,593,302, dated Jun. 3, 1986, in the names of Alexander Lidow and Thomas Herman.
When MOSFETs are subjected to radiation, several of their characteristics are modified and degraded. For example, ionizing radiation is known to induce charges into the gate oxide, which produces a shift in gate-to-source threshold voltage. Gate-to-source threshold voltage decreases with increasing total radiation dose for N channel devices, and increases with total dose for P channel devices. The gate drive circuitry must be designed to offset these threshold voltage shifts by overriding them with appropriate biasing levels. This complicates the control circuitry. A description of the shift in threshold voltage is described in more detail in a paper entitled "Radiation Resistance of HEXFETs," contained at pages B-10 through B-12 of the HEXFET Databook of 1985, published by the International Rectifier Corporation of El Segundo, Calif.
A power MOSFET having a more constant threshold voltage for a total radiation dose, up to 1 megarad, would be very desirable since it would simplify the gate drive circuitry.
In addition to the total dose dependent characteristics which are mentioned in the above reference, it is also known that the device breakdown voltage can degrade. A power MOSFET having a more constant breakdown voltage for a total dose, up to 1 megarad, would also be desirable because the device breakdown voltage need not be derated as much. This may result in a device selection which has a lower on resistance for the designer.
The shift in gate-to-source threshold voltage described above occurs at all dose rates. Power MOSFET applications in free space environments are particularly susceptible to radiation induced threshold shifts. At higher dose rates, for example, 1.times.10.sup.9 through 1.times.10.sup.13 rads/second, a failure mode appears, termed burnout, which is like avalanche energy failure. In this failure mode, the parasitic bipolar transistor of one or more cells of a multicellular vertical conduction MOSFET (which has thousands of parallel cells in a common chip) appears to have turned fully on and then hogs all current flow through the device until it is destroyed. Such high dose rates can be produced by nuclear explosions which, for example, may generate ionizing radiation at 10.sup.12 rads/second.
It would be very desirable to prevent the burnout effect due to a high ionizing dose rate.
A further source of device degradation due to radiation is that caused by a neutron flux. Neutrons cause physical damage to the silicon body of the device 10 which increases the on-resistance of the device. This effect is greater with higher resistivity silicon. Since higher rated voltage devices are produced by using higher resistivity silicon, the effect is most pronounced in higher voltage rated devices. Thus, devices rated at 100 volts, employing 1.5 ohm/cm material are only slightly affected, while devices rated at 400 volts, employing 15 ohm/cm material show a two and one-half fold increase in on resistance for a neutron fluence of 10.sup.14 neutrons/cm.sup.2.
It would be very desirable to provide a high power MOSFET which has a relatively high voltage rating but will not have a too greatly degraded on resistance when exposed to a high neutron flux.